SECT, in.] CONDENSATION OF STEAM. 133 



already on fire, as that would instantly send out a volume of smoke. In this 

 situation they were gradually dried, and the smoke which issued from them con- 

 sumed by the current of air from the fire door, in passing over the bright burning 

 fuel. The opening or openings, to admit the air, are regulated so as just to admit 

 the quantity which consumes the smoke ; more would be prejudicial. He at first 

 constructed these furnaces in a rather different manner; but found the above 

 method the most convenient, and, when properly attended to, answers the purpose 

 perfectly with free burning coals, but is more difficult to manage with coal 

 which cakes. 



247. Robertoii's Furnace. Various methods of construction have been contrived 

 to accomplish the objects proposed by Mr. Watt : that of Messrs. Roberton is 

 perhaps on the whole the best. The opening through which the fuel is intro- 

 duced into the furnace is shaped somewhat like a hopper, and is made of cast 

 iron, built into the brickwork, inclining from the mouth downward to the place 

 where the fire rests on the grate. The coals in this mouth-piece or hopper answer 

 the purpose of a door, and those that are lowest are by this means brought into 

 a state of ignition before they are forced into the furnace. Below the lower plate 

 of the hopper the furnace is provided with upright front bars, which serve to admit 

 air among the fuel, and to admit an implement to force the fuel back, from time 

 to time, to make room for fresh quantities to fall into the furnace from the hopper. 

 By this arrangement the fuel is brought into a state of ignition before it reaches 

 the further end of the bottom grate, where it is stopped by the rising breast of the 

 brickwork, so that any smoke liberated from the raw coals at the front must pass 

 over these red hot coals before it can reach the flue. 



Below the upper side of the mouth-piece or hopper, and about the distance of 

 three-fourths of an inch from it, is introduced a cast iron plate. This plate is 

 above the fuel, and the space between it and the top of the hopper is open to admit 

 a very thin stream of air, which, rushing down the opening, comes first in contact 

 with that part of the fire which is giving off the greatest quantity of smoke, mixes 

 with it before it passes over the hot fuel in the interior, and therefore in passing 

 it inflames and escapes undecomposed. This is the worst part of the apparatus ; 

 for air so admitted cools the bottom of the boiler. 



The quantity of air admitted to pass over the upper surface of the fire is 

 regulated by inserting a wedge-formed piece of iron. The front bars are closed 

 by doors, which, when shut, prevent the heat from coming out, and incommoding 

 the workmen. 



248. A considerable improvement was added by Mr. Woolf, to enable them 

 to get rid of clinkers and scoriae. The contrivance is extremely simple. The com- 

 bustion of the fuel commences, and is chiefly carried on, on the part of the bottom 



